Inorganic-organic nanocomposites often exhibit properties exceeding those of macroscopic composites. Typically, the inorganic component is a modified clay and the organic component is a polymer. While the definition of a nanocomposite is still a subject of debate, a generally accepted working definition for inorganic-organic nanocomposites is a composite in which the inorganic phase is no larger than 1 micron in size and the organic (polymeric) phase is continuous, that is, nanocomposites are highly dispersed systems of submicron-sized inorganic particles in a polymeric matrix. Such nanocomposites are not to be confused with ordinary composites of an unmodified clay (i.e., a clay in its natural state), such as mica or talc, and a polymer, where the clay simply acts as a filler for economic reasons, with negligible impact on physical properties.
To illustrate, nanocomposites made from alkyl quaternary ammonium modified bentonite clays and polyamides possess higher heat deflection temperature, greater toughness, and reduced moisture permeability, making them useful in automotive timing belts. Exemplary disclosures of nanocomposites include Kawasumi et al., U.S. Pat. No. 4,810,734 (1989); Usuki et al., U.S. Pat. No. 4,889,885 (1989); Okada et al., U.S. Pat. No. 4,894,411 (1990); and Maxfield et al., U.S. Pat. No. 5,385,776 (1995).
The modified clay is derived from a naturally occurring layered clay (e.g., bentonite or hectorite) in which negatively charged layered structures or sheets are held together by sodium, potassium, magnesium or other inorganic cations sitting in anionic galleries between the sheets. There may be a further layered structure within each sheet; for example silicate clays are characterized by a center layer of aluminum and magnesium hydroxides fused between two layers of silica. The sheet-to-sheet separation (or interlayer distance) is on the order of 15 .ANG.. A layered clay in this natural state is unsuitable for the preparation of nanocomposites because it is too hydrophilic and the layers are held together too tightly by the inorganic cations to interact with the hydrophobic polymer molecules and be dispersed thereby. To make it more polymer-compatible, a layered clay is modified by replacing the inorganic cations with organic cations, with the resulting product being referred to as a "modified layered clay" or an "organoclay." Commonly, the organic cation is a quaternary ammonium cation. The replacement increases the interlayer distance to 25 .ANG. or more, reduces the interlayer forces, and makes the clay more hydrophobic, rendering it amenable to dispersion in a polymer.
Modified layered clays also can be used for other purposes, such as thickeners for greases and polyester laminating resins and are available from suppliers such as Rheox Inc. or Southern Clay Products.
In one method, a nanocomposite is prepared by dispersing the modified layered clay in a polymerizable liquid monomer followed by polymerization of the liquid monomer in the presence of the modified layered clay. Polymerization is effected by UV light, acid, base, or heat, and the polymerization temperature typically is below 200.degree. C. and is necessarily below 250.degree. C. to avoid decomposition of the modified layered clay. A limitation of this method is that the polymer must be produced from a liquid monomer. Alternatively, a nanocomposite is prepared by melt blending a modified layered clay with a polymer. In this case, the modified layered clay needs to be stable at the melt processing temperature of the polymer. Hitherto, it has not been feasible to make a modified layered clay nanocomposite with a melt processed polymer such as a crystalline thermoplastic having high crystalline melting temperature (e.g., a fluoroplastic) or an amorphous polymer having a high glass transition temperature, because a conventional quaternary ammonium cation modified layered clay is stable only up to about 250.degree. C.